Practical application of the superconducting phenomenon has long been sought because it offers the following characteristics: it reduces electric resistance to zero, and it exhibits diamagnetism, which can help to greatly reduce electric power loss. Previously, it was believed that the superconduction phenomenon could only take place under cryogenic conditions. However, following the developments of new superconductors made using intermetallic compounds, the critical temperature, which is treated as a parameter of the practical application of the superconduction phenomenon (the boundary temperature between normal conduction and superconduction at which electric resistance drops substantially) has risen from year to year, and the superconduction phenomenon can now occur at normal temperatures.
A great deal is expected from the practical application of this superconduction phenomenon in the semiconductor device field as well, and there have been several proposals for high-speed, low-power-consumption superconducting transistors capable of realizing low power loss. Of these types of transistors, a voltage-driven superconducting transistor has drawn the most attention because of the ease with which it can be driven due to its large input impedance and its diminished input loss.
FIG. 5 shows a cross section illustrating the structure of a conventional voltage-driven superconducting transistor. This figure reveals that the superconducting transistor uses monocrystals of silicon as a substrate 14, on which an n-type semiconductor region 15 is formed by using arsenic ions. This semiconductor region 15 is connected to a source 11 and a drain 12, which act as superconducting electrodes, and a gate 13 insulated by a gate oxide film 18 is disposed between the source 11 and the drain 12, the gate 13 being covered by a side-insulating film 17 and an overhang 16. For example, source 11 and drain 12 are formed by niobium (Nb), the gate 13 is formed by polycrystalline silicon, the overhang 16 and the side-insulating film 17 are formed by Si.sub.3 N.sub.4, and the gate oxide film 18 is formed by SiO.sub.2.
In a superconducting transistor with such a configuration, Cooper pairs exude from the source 11 and the drain 12 as much as a coherence length, which is modulated by voltage applied to the gate 13 in order to link the source 11 with the drain 12 using the Cooper pairs. Since the coherence length is several dozens of nm (e.g., 24) in conventional superconductors, a superconducting transistor with the above configuration requires the distance between the gate 13 and the drain 12 to be reduced to about 0.1 .mu.m.
In addition to the voltage-driven superconducting transistors using superconductors, current-injected superconducting transistors have also been proposed. However, due to current injection, these transistors suffer from problems such as heat generation and small current gain, and there have been no reports claiming the realization of good characteristics.
Because conventional voltage-driven superconducting transistors have a short coherence length as described above, it has been necessary to reduce the gate length (the distance between the source and the drain) to 0.1 .mu.m or less. The relation between the coherence length .lambda. and a superconduction gap .phi. in a voltage-driven superconducting transistor may be expressed approximately by the following formula (1) according to the BCS theory: EQU .lambda..infin. (1/.phi.) (1)
Since the above superconduction gap .phi. is proportional to the critical temperature (T.sub.c) of a superconductor, the coherence length .lambda. becomes shorter as the critical temperature (T.sub.c) rises. Therefore, in a superconducting transistor that utilizes oxides for the superconductor, the coherence length .lambda. of the oxide superconductor with a critical temperature (T.sub.c) of 40K or higher is several nm or less, which means that the gate length must be reduced further. It is extremely difficult to manufacture a device with such a short gate length, and the increased proximity of the source and the drain causes the element withstand voltage to decreases, which creates a problem.
Accordingly, in order to solve these problems, the applicant has proposed a superconducting element with a structure as shown in FIG. 6. The superconducting transistor shown in FIG. 6 (a) uses SrTiO.sub.3 (hereinafter abbreviated as STO) as a substrate 1, on which a barrier layer 3 composed of PrBa.sub.2 Cu.sub.3 O.sub.7-x (hereinafter abbreviated as PBCO) is formed.
Subsequently, an oxide superconductor, YBa.sub.2 Cu.sub.3 O.sub.7-x (hereinafter abbreviated as YBCO) is formed on the barrier layer 3, and then a gap 4 on the order of submicrons is formed and used as a source region 2a and drain region 2b. In addition, a gate electrode 6 composed of an STO film is formed on the gap 4 via an insulation layer 5. Furthermore, a source electrode 7 on the surface of the source region 2a is made of Au, and a drain electrode 8 made of Au is also formed on the surface of the drain region 2b.
A superconducting transistor with such a configuration resolves the restriction in the gate length from the coherence length allowing the realization of a superconducting transistor that has a large distance between the source and the drain, as well as excellent withstand voltage, by using a PBCO film as the barrier layer 3, which connects the source region 2a to the drain region 2b. The PBCO film has a construction similar to the crystalline structure of a YBCO film, an oxide superconductor. In other words, a superconducting transistor with this configuration has a source drain 2a that is connected to the drain region 2b where the superconducting current flows due to carriers accumulated at the interface of the barrier layer 3, which corresponds to the gap 4 between the source region 2a) and the drain region 2b. The carriers are injected from the YBCO film into the PBCO film, which becomes semiconductive because of its low carrier concentration even though it has a crystalline structure similar to that of the YBCO film.
However, in the above-described superconducting transistor, the lines of electric force (g) in the vicinity of the YBCO film (the source region 2a and the drain region 2b) among the lines of electric force (g) due to the gate voltage are not irradiated vertically from the gate electrode 6, but rather are attracted to the sides of the YBCO film, which has an equipotential surface, as shown in FIG. 6 (b). This causes the electric fields on the source region 2a side and the drain region 2b side to decrease in the interface on the barrier layer 3 corresponding to the gap 4 between the source region 2a and the drain region 2b, and this decrease, in turn, causes the carriers attracted to these portions to decrease, making it difficult to bond the source region 2a and the drain region 2b smoothly. Therefore, in order to have the superconduction current flow between the source and the drain, it is necessary to apply a gate voltage that is sufficiently large to account for the loss in the lines of electric force (g) that are absorbed into the YBCO film, thereby creating a problem in that the current gain becomes smaller relative to the voltage between the source and the drain, as well as the gate voltage.
Therefore, given the above problems, the present invention is intended to provide a superconducting transistor (a superconducting device) that has excellent current output characteristics and can uniformly draw out the carriers relative to the gate voltage onto the interface of the barrier layer without disturbing the lines of electric force (the electric field) created by the gate voltage.